1. Technical Field
The invention relates to an optical unit for scanning an information plane having an optically readable and track-wise arranged structure, which unit comprises a radiation source for supplying a radiation beam, an objective system for focusing the radiation beam to a scanning spot in the information plane, an adjustable element arranged between the radiation source and the objective system for moving the scanning spot transversely to the track direction, a radiation-sensitive detection system for converting the radiation beam from the information plane into electric signals, and a position detection system for determining the position of the adjustable element.
2. Related Art
An optical unit of this type can be used in an optical disc player, such as the well-known CD or CD-ROM players and optical players based on the principle of these players, to guide a radiation beam to an information plane of an optical record carrier and from the record carrier to a radiation-sensitive detection system. If the record carrier is provided with user information, such as an audio program or a video program, the radiation beam from the radiation source is an unmodulated beam and this beam is modulated by the information structure in the information plane. The modulated beam from the record carrier is converted by the radiation-sensitive detection system into an electric signal representing the information which is being read out. If the record carrier is a blank record carrier, the beam from the source is modulated with information to be written and the radiation-sensitive detection system is used only for deriving servosignals.
One of the great advantages of a disc system is that the recorded information is random accessible. The scanning spot can be positioned on each arbitrary track by moving the optical unit and the disc with respect to each other in the radial direction, i.e. a direction transverse to the track direction. Because of the small weight of the optical unit, such a movement can be performed at a relatively fast rate. For new applications of the optical disc, it becomes necessary to further reduce the access time. It has already been proposed to realize faster access by inserting a movable element, for example, a mirror which deflects the beam over the aperture of the objective system so that the spot formed by the objective system is displaced transversely to the track direction. Use is then made of the fact that the objective system has an image field which is relatively large compared to the diameter of the spot, so that the spot still has a good quality in the case of a moderate movement of the spot within this field. In order to maintain the high spot quality, i.e. to keep the axis of the radiation beam close to the center of the objective system, during a fast radial movement of the spot, the objective system is also displaced, but at a smaller rate than the mirror. The displacement of the objective system should be related to the deflection of the beam by the mirror, so that the optical unit should comprise a position detection system for determining the angular position of the mirror.
An optical read/write unit comprising such a position detection system is known per se, for example, from U.S. Pat. No. 4,423,496 to Opheij et al. This patent relates to an improvement of the tracking servosystem for maintaining the center of the scanning spot on the center line of the track being scanned. A tracking error signal, needed for a spot position correction, is obtained by projecting the radiation beam from the information plane on two detector elements, which occupy different radial positions with respect to the information track structure, and by subtracting the output signals of these detector elements. The tracking error signal is supplied to the drives of a pivotable mirror. This mirror deflects the radiation beam so as to fine-position the center of the spot on the center line of the track being scanned. However, movement of the mirror causes a change of the radiation intensity distribution on the position detector elements, which change is independent of the position of the spot with respect to the track being scanned. The tracking error signal is thus dependent on the angular position of the mirror. To eliminate this dependency, a position detection system is used to determine the angular position of the mirror, and the output signal of this system is combined with the tracking error signal to correct the tracking error signal and make this signal independent of the mirror position. The mirror position detection system described in U.S. Pat. No. 4,423,496 uses either a separate position-sensing beam reflected at the rear side of the mirror and two detection elements arranged at this rear side, or a portion of the radiation beam transmitted by the pivotable mirror, or a portion of the radiation beam reflected by a surface in front of the mirror. In all of the embodiments of this mirror position detection system, the position detection elements are arranged in a plane which is different from the plane of the radiation-sensitive detection system.
It is an object of the present invention to provide a fast-access optical unit, wherein the range of movement of the movable element is minimum and the signal from the position detection system is used to control the movement of at least the objective system by means of a dedicated and attractive position detection system.
This optical unit is characterized in that the position detection system comprises a weakly reflecting element arranged in the path of the radiation beam between the adjustable element and the objective system for reflecting a portion of the radiation beam as a position-sensing beam towards a radiation-sensitive position detector comprising at least two detector elements.
Not only the function, but also the construction of this position detection system is different from that described in U.S. Pat. No. 4,423,496.
The optical unit is also characterized in that the normal to the reflecting plane of the weakly reflecting element extends at a small angle to the optical axis of the objective system.
By this measure, the position-sensing beam is spatially separated from the radiation beam.
The detection system is further characterized in that the position detector is arranged in the same plane as the radiation-sensitive detection system.
The position detector and the detection system may then be integrated in one component so that the number of components of the optical unit as well as alignment problems can be reduced.
A first embodiment of the optical unit is characterized in that the reflecting element is a weakly reflecting flat plate, in which the normal to the plate surfaces extends at a small angle to the optical axis of the objective system.
A second embodiment of the optical unit is characterized in that the reflecting element is a weakly reflecting wedge having a small wedge angle.
A third embodiment of the optical unit, which comprises a xcex/4 plate arranged between the movable element and the objective system, is characterized in that the weakly reflecting element is constituted by an uncoated surface of the xcex/4 plate, which surface faces the objective system.
The xcex/4 plate may be used in an optical unit wherein the radiation beam delivered by the radiation source is linearly polarized, either to prevent radiation from being coupled back to the radiation source or, in combination with a polarizing beam splitter, to separate the beam reflected by the information plane from the radiation beam to the record carrier.
By using the xcex/4 plate as the weakly reflecting element, the number of components of the optical unit is reduced to a minimum. Optimum use is thus made of the facts that an uncoated surface has a reflection of the order of 4% and that the position-sensing beam needs to have only a low intensity. It also suffices for the previous embodiments when one of the surfaces of the weakly reflecting element is uncoated, so that this element does not need to have a beam-separating layer.
The above embodiments may be further characterized in that the movable element is a plane mirror.
The invention also relates to an apparatus for reading information from and/or writing information in an information layer of an optical record carrier, which apparatus comprises supporting means for the record carrier, an optical unit for supplying a scanning beam focused to a scanning spot on the information layer, and means for moving the record carrier and the optical unit relative to each other. This apparatus is characterized in that the optical unit is a unit as described above.